1. Field of the Invention
The present invention relates to a fixing apparatus in which a toner image is fused by heat produced by electromagnetic induction.
2. Description of the Related Art
A conventional image-forming apparatus that uses toner to form images performs an electrophotographic image-forming process such as electrophotography, electrostatic recording, and magnetic recording. A toner image is transferred directly or indirectly onto a surface of a print medium and then the toner image is subsequently fused by heat. A fixing unit for this purpose incorporates a pressure roller and a metal fixing roller. A halogen lamp is used as a heat source that heats the metal fixing roller to a predetermined temperature. When a print medium passes a nip formed between the fixing roller and the pressure roller, the toner image is fused by heat to the print medium. In this case, because the fixing roller has a large heat capacity, a long time is required for heating the fixing roller to a predetermined temperature. Another type of fixing apparatus is one in which one surface of a thin film is in contact with a heat source such as a ceramic heater having a small heat capacity and the other surface of the thin film is in contact with the print medium. The thin film has small heat capacity, good heat-resistance, and excellent heat conduction. This configuration allows a quick start of the fixing apparatus, thereby permitting a saving of power of the image-forming apparatus and preventing an increase in interior temperature of the image-forming apparatus.
Still another type of fixing apparatus uses a heater of electromagnetic induction type. An electromagnetic induction type fixing apparatus supplies joule heat, generated by an eddy current that flows in a heating element, to a print medium to fuse a toner image.
FIG. 19 illustrates an example of a conventional apparatus employing an electromagnetic heater element. A magnetic flux generator 3 generates a time-varying magnetic flux. The magnetic flux causes an eddy current by electromagnetic induction and the eddy current generates heat. A heater element 2 is inscribed in a film-like fixing roller having a low heat capacity. The toner 5 transferred onto a print medium 6 passes through a nip formed between the fixing roller 1 and a pressure roller 7. The toner 5 receives heat from the heater element 2 through the fixing roller 1 to be fused to the print medium 6. A non-contact type temperature sensor 4 detects the temperature at the nip.
FIGS. 20A and 20B are cross-sectional views of the conventional fixing apparatus when it is seen from an upstream side of the fixing apparatus with respect to the direction of travel of the print medium. For ease of explanation of heat distribution on the fixing roller 1, the pressure roller 7 is not shown in FIGS. 20A and 20B. The heater element has a width that is enough to evenly fix a toner image on a maximum-width print medium 6b. The temperature sensor 4 is in contact with the heater element 2 and disposed in a longitudinal direction substantially at a center of the nip, thereby reliably detecting the temperature of the heater element 2 regardless of which one of a minimum-width print medium 6a and a maximum-width print medium 6b passes through the nip.
FIG. 20A illustrates the temperature profile on the fixing roller 1 when the maximum-width print medium passes the nip. FIG. 20B illustrates the temperature profile on the fixing roller 1 when the minimum-width print medium passes the nip. In accordance with the temperature detected by the temperature sensor 4, a CPU 10 controls a power supply 9 to maintain the nip area at a predetermined temperature. The heater element 2 is designed to generate heat so that when the maximum-width print medium 6a passes through the nip, the toner image on the maximum-width print medium 6a can be fused evenly. The heater element 2 generates an amount of heat uniformly distributed along the length of the heater element 2. Therefore, when the print medium 6 having a smaller width than the maximum-width print medium 6a passes through the nip, heat is also generated in areas H of the heater not in contact with the print medium 6. However, the heat generated in the areas H is not transmitted to the print medium 6 and the toner image on the print medium 6.
As a result, the temperature is higher at the areas of the heater element 2 not in contact with the print medium 6 than at the areas in contact with the print medium 6.
The heat generated at the areas H causes an excess increase in temperature in the apparatus, which in turn shortens the lifetime of the pressure roller and the film-like fixing roller. This is also detrimental from a point of view of energy saving.